Conventionally, there has been suggested a technical method for detecting white lines or other lane marks from a road surface image ahead of a vehicle captured by a camera mounted on the vehicle (for example, refer to Japanese Patent Laid-Open No. Hei 11(1999)-147481 and Japanese Patent Laid-Open No. Hei 11(1999)-147473). The detection result of the lane marks is used to control, for example, a lateral position of the vehicle with reference to the positions of the lane marks located on both sides of the vehicle.
According to the conventional technology, the white lines are detected by utilizing a property that the luminance of the white lines included in the road surface image is higher than those of other portions of the road surface.